This invention relates to a service vessel operating method and to a service vessel for applying the operating method.
The main propulsion engine of a ship is typically a diesel engine. Generally, the operating speed of the engine is constant and is the speed at which the power output is maximum. It is known that at low speeds the propulsion force of a ship or vessel can be increased by using nozzle propellers, wherefore nozzle propellers are generally used in so-called service vessels, especially tow-boats (tugs). Nozzle propellers have also been used in vessels designed for navigation in ice. However, ice tends to block or obstruct the nozzle, and this causes flow disturbances in the propeller leading to a weakening of the propulsion force and to severe vibrations. Attempts have been made to lessen these problems by specific designs of the bottom of the ship and by using additional devices, but these attempts have not generally produced noteworthy results.
The object of this invention is to provide an operating method by which the above mentioned disadvantages are avoided and which provides for an efficient use of a service vessel in a reliable manner and with simple means.